1. Field of the Invention
The present invention relates to an optical disc, an apparatus for initializing and a method of manufacturing an optical disc, and an initialization reference plate, and is suitably applicable, for example, to an optical disc apparatus which records a plurality of recording marks on a single recording layer in a thickness direction.
2. Description of the Related Art
Optical disc apparatuses that irradiate an optical disc such as compact disc (CD), digital versatile disc (DVD), and Blu-ray Disc (registered trademark, referred to as BD, hereinafter) with a light beam and read the reflected light for information reproduction have been in widespread use.
Such hitherto known optical disc apparatuses also perform information recording by irradiating the optical disc with a light beam so that the optical disc causes a local change in reflectance etc.
For the optical disc, it is known that a light spot of approximately λ/NA (λ: the wavelength of the light beam, NA: numerical aperture) in size is formed on the optical disc, with resolution proportional to this value. For example, an optical disc of BD type having a diameter of 120 mm can record data as much as approximately 25 GB.
Optical discs are to record a variety of information including various types of contents such as music contents and video contents and various types of data such as computer data. In particular, videos have gotten higher in resolution and music higher in quality recently with increased amounts of information, and the number of contents that are desired to be recorded on a single optical disc has been on the increase. A further enhancement in capacity has thus been required of the optical discs.
Consider now the case of providing a plurality of recording layers in a single optical disc. As shown in FIG. 1A, if the optical disc is formed by stacking different types of materials like reflecting layers and recording layers as in the hitherto known DVD and BD systems, the complicated manufacturing steps can increase the manufacturing cost etc.
Some optical disc apparatuses that have been proposed for the sake of increased capacity record a plurality of layers of recording marks (hereinafter, referred to as mark layers) as if stacked in a uniform recording layer of an optical disc (refer to, e.g., Jpn. Pat. Appln. Laid-Open Publication No. 2008-071433).
As shown in FIG. 1B, an optical disc DK corresponding to an optical disc apparatus of such configuration has a recording layer DKw that is uniform inside, without any possible position guideline in the recording layer. The optical disc DK is thus provided with an additional reflecting layer DKs that includes servo tracks or the like.
More specifically, the optical disc apparatus focuses a predetermined servo light beam LS on a servo track of the reflecting layer DKs to be irradiated with the servo light beam LS (hereinafter, this servo track will be referred to as a desired servo track TRG). The optical disc apparatus further focuses a light beam LB having an optical axis coincident with that of the servo light beam LS upon an intended position (hereinafter, referred to as a target mark position PG) in the recording layer DKw.